Sydney: Sydney's iconic Harbor Bridge and Opera House went dark today as Australians switched off lights around the country for an hour as part of a global effort to shine a spotlight on climate change.

Hundreds of landmarks around the world including Washington's National Cathedral, London's Clock Tower, the Great Wall of China and Tokyo Tower will be dimmed at 8:30 pm local time.

The central Sydney icons have been taking part in the annual event since Earth Hour began as a Sydney-only event in 2007. Australia is among the first countries to hit the light switches each year.

In New Zealand, Sky Tower in Auckland and parliament buildings in Wellington switched off two hours earlier.

WWF, the Washington-based environmental group that organizes the event, said the number of countries and territories participating had grown from 135 last year to 147 this year.

Libya, Algeria, Bhutan and French Guinea are among those participating for the first time.

"Earth Hour 2012 is a celebration of people power; the world's largest mass event in support of the planet," WWF official Dermot O'Gorman told reporters in Sydney on Saturday, hours before the event.